1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) transmitter, a CDMA multiplex transmitter, a CDMA receiver and a CDMA communication system, which can, for example, be applied as a fiber-optic access system using a fiber-optic CDM (Code Division Multiplexing) technique and a PON (Passive Optical Network).
2. Related Background Art
Presently, several CDMA systems are available, including a synchronous CDMA system that obtains synchronization for each channel, an asynchronous CDMA system that does not obtain synchronization for each channel, and a quasi-synchronous CDMA system that, to a degree, permits differences in synchronized positions. When an asynchronous CDMA system or a quasi-synchronous CDMA system is employed, compared with the length (the number of chips) of code, the number of multiplexed data that can be obtained is not large. Therefore, a synchronous CDMA system, with which an equal number of chips and of multiplexed data sets can be obtained, is an appropriate CDMA communication system for the performance of multiplex transmissions.
However, since for the demodulation of multiplexed data a synchronous CDMA system requires a sync signal, in addition to an original transmission signal, a sync signal must also be transmitted.
Further, for a synchronous CDMA system employed by a radio communication system that conforms to the IS-95 standards, two codes, such as a PN (Pseudo-Noise) code and an orthogonal code, are jointly used to synchronize individual channels for transmission, with one of the codes, e.g., the PN code, being employed as a sync signal.
However, for a CDMA communication system that uses a conventional synchronous CDMA system, the reception side not only requires a despreading device, for fetching a transmission signal, but also a coherent detector, for detecting a sync signal. Thus, when a hardware arrangement is used for the two devices, the circuit size is increased, and when software is used to implement the functions of these devices, an enormous number of processing steps is required.
For a CDMA communication system embodying a synchronous CDMA system that employs two codes, one of which is a sync code, the reception side requires two correlation devices, such as a PN code correlation device and an orthogonal code correlation device, to detect the two codes, and when hardware is used to prepare an arrangement of the two devices, the size is increased, while when software is used to implement these devices, an enormous number of processing steps is required.
Therefore, a demand has arisen for a CDMA transmitter, a CDMA receiver, a CDMA multiplex transmitter and a CDMA communication system having a simple configuration for performing, on the reception side, a coherent detection process, or for, also on the reception side, a coherent detection setup for which only a small number of processing steps are required.